1. Field of the Invention
The present invention relates to a method and apparatus for implementing an inband secondary channel in a voiceband modem and using this secondary channel for achieving synchronization in single port and multiport voiceband modems.
2. Background
It is frequently desirable to provide data modems with the capability to communicate over a secondary channel to implement diagnostics and network control functions. U.S. Pat. No. 4,385,384 to Rosbury et al. describes a method and apparatus for implementing an out of band FSK secondary channel by frequency division multiplexing a low speed frequency shift keying channel with the main channel to provide simultaneous communication over the same line as main channel data. Unfortunately, this solution is less practical for very high speed modems operating for example at 19.2 Kbps over voiceband channels. Such high speed modems require virtually all of the bandwidth of the channel and the low speed frequency shift keying transmitter is relatively inefficient in it's use of bandwidth. Also, there is a hardware penalty for using a separate secondary channel transmitter and receiver. The present invention mitigates these problems with an improved arrangement for implementing an inband secondary channel.
Inband techniques have also been used to provide secondary channel capability in data modems. In U.S. Pat. No. 4,627,077 to Armstrong and U.S. Pat. No. 4,630,287 also to Armstrong describe methods for shifting the position of a main channel data symbol in order to encode secondary channel data. A similar technique is disclosed in U.S. Pat. No. 4,644,537 to Gitlin et al. wherein four main channel data symbols are represented by either of two constellation points. One of the two constellation points represents a secondary channel "1" and the other represents a secondary channel "0". These techniques, although using no more bandwidth than the main channel data and providing little main channel degradation, provide a secondary channel which is no more robust (i.e., resistant to errors in the data in the presence of noise and other disturbances) than the main channel. Consequently, if the error performance of the main channel is unacceptable, the secondary channel error performance will also be unacceptable. Thus, some of the diagnostic functions which are desirable to implement in a secondary channel cannot be carried out when line conditions are poor and perhaps when they are needed most.
A similar inband technique is disclosed in U.S. Pat. No. 4,525,846 Bremer et al. In this patent, a secondary channel is implemented by encoding the secondary channel bits by changing the radial distance of the constellation from the origin. This provides a data rate for the secondary channel which apparently is determined by the statistics of the main channel data. In U.S. Pat. No. 4,509,171 also to Bremer et al., a similar radial modulation scheme is used to provide multiport synchronization by detecting the amplitude difference in two main channels. In U.S. Pat. No. 4,389,722 to Hofmeister amplitude modulation is used to facilitate identification of the various channels in a modem transmitting a plurality of data streams. In U.S. Pat. No. 4,347,616 Murakami, frame synchronization bits are added to the signal and modulated with a predetermined detectable amplitude to facilitate multiport synchronization.
In U.S. Pat. No. 4,227,152 to Godard et al., a method of equalizer training is disclosed in which an equalizer is trained only on outer points of a constellation or only in predefined regions of points of the constellation.